This invention relates to a flexible printed circuit (FPC) including a means for decreasing electromagnetic noises radiated from the flexible printed circuit and, more particularly, to electromagnetic noises protection of the flexible printed circuit for use in electrically connecting between a body (a fixed portion) of an optical disc drive and an optical pickup unit (a movable portion), contained therein and including an electromagnetic radiation source, that records/reproduces data in/from an optical recording medium (an optical disc) such as a CD-R (compact disc-recordable), a CD-RW (CD-rewritable), a DVD (digital versatile disc) or the like.
In the manner known in the art, a flexible printed circuit is a circuit for use in electrically connecting between a fixed portion and a movable portion. For example, when the fixed portion comprises a body of an optical disc drive, the movable portion comprises an optical pickup unit contained therein. The optical pickup unit is disposed so as to be opposed to an optical disk rotating with a predetermined distance left therebetween and so as to slidably move against the body of the optical disc drive.
The optical pickup unit is a unit which carries out recording (writing) or erasing of information by converging a laser beam produced by a semiconductor laser serving as an optical source on a signal recording face of an optical disc through an objective lens and which carries out reproducing of information by detecting a reflected beam (a return beam) from the signal recording face by a photodetector serving as optical detecting means.
Inasmuch as it is necessary to carry out transmission/reception of information between the optical pickup unit serving as the movable portion and the optical disc drive""s body serving as the fixed portion, therebetween are electrically connected by the flexible printed circuit. For instance, signals for transmitting from the optical disc drive""s body to the optical pickup unit may be a signal for driving a semiconductor laser, a current flowing in coils of an actuator for driving the objective lens in a focus direction and a tracking direction. On the other hand, signals for transmitting from the optical pickup unit to the optical disk drive""s body may be a signal detected by the photodetector or the like.
Now, there is the optical pickup unit including an electromagnetic radiation source mounted thereon. On the other hand, the flexible printed circuit comprises a flexible base layer on which a plurality of signal lines are formed. Each signal line in the flexible printed circuit serves as an electromagnetic antenna. That is, radiated from the electromagnetic radiation source, electromagnetic noises are transmitted on the signal lines and are radiated from the signal lines to the outside.
In the optical pickup unit including the electromagnetic radiation source mounted thereon, it is important to provide electromagnetic interference (EMI) protection because the electromagnetic noises affect electronic apparatuses each other. If any EMI protection is not provided for the flexible printed circuit, it is impossible to shield the electromagnetic noises because each signal line in the flexible printed circuit acts as the electromagnetic antenna.
In order to shield the electromagnetic noises, a conventional flexible printed circuit in which an EMI protection is provided comprises a ground layer. However, in order to form the ground layer, it is necessary for the flexible printed circuit to make multiple layers. As a result, the conventional flexible printed circuit where the EMI protection is performed is disadvantageous in that additional cost is incurred in the flexible printed circuit.
It is therefore an object of this invention to provide a flexible printed circuit which is capable of shield electromagnetic noises.
It is therefore another object of this invention to provide a flexible printed circuit of the type described, which has a low cost.
Other objects of this invention will become clear as the description proceeds.
On describing the gist of this invention, it is possible to be understood that a flexible printed circuit is for electrically connecting between a fixed portion and a movable portion. The flexible printed circuit comprises a flexible base layer having a principal surface and a lower surface. A conductive layer is formed on the principal surface of the flexible base layer through a first adhesive agent. A cover lay is formed on the conductive layer through a second adhesive agent. The cover lay has an upper surface.
According to an aspect of this invention, the above-mentioned flexible printed circuit further comprises foil made of conductive material pasted on at least one of the lower surface of the flexible base layer and the upper surface of the cover lay through a third adhesive agent. The foil is electrically connected to a ground portion.
In the afore-mentioned flexible printed circuit, the foil may comprise copper foil. The third adhesive agent preferably may comprise a conductive adhesive agent. Electrical connection between the foil and the ground portion may be made using solder. The conductive layer comprises various signal lines including a power supply line and a ground line. The foil may be pasted on the at least one of the lower surface of the flexible base layer and the upper surface of the cover lay so as to cover at least the power supply line and the ground line.
In the afore-mentioned flexible printed circuit, the fixed portion may be a body of an optical disc drive. The movable portion may be an optical pickup unit comprising an electromagnetic radiation source. In this event, the foil is for shielding electromagnetic noises radiated from the electromagnetic radiation source. The foil may be disposed only the vicinity of the electromagnetic radiation source. The conductive layer comprises various signal lines including a power supply line and a ground line. The foil may be pasted on the at least one of the lower surface of the flexible base layer and the upper surface of the cover lay so as to cover at least the power supply line and the ground line.